Self-Assembly Processes for Organic LED Electrode Passivation and Charge Injection Balance

Abstract

primary colors by varying the size of the arrays of metal nanoparticles (width, length), or by using different metals. A more detailed study concerning color tuning of these films by selecting proper materials and processing conditions will be presented separately. ExperimentalColloidal silver particles with a surface layer of dodecanethiol were synthesized according to slightly modified versions of methods reported in the literature [20,21]. Silver trifluoromethanesulfonate (CF 3 SO 3 Ag, 9 × 10 ±4 mol), tetraoctylammonium bromide ([C 8 H 17 ] 4 NBr, 4 × 10 ±3 mol), and 1-dodecanethiol (C 12 H 25 SH, 1.8 × 10 ±3 mol) were dissolved in 80 mL of toluene. A freshly prepared aqueous sodium borohydride solution (1 × 10 ±2 mol in 25 mL of distilled water) was slowly added under vigorous stirring. The organic layer was separated, and the solution was concentrated by solvent evaporation to a volume of approximately 5 mL. Then, 500 mL of ethanol was added at ±20 C. After sedimentation of the particles, the outstanding solution was decanted, and the residue was again treated with 500 mL of ethanol at ±20 C. Finally, the particles were dried for 12 h at room temperature and under reduced pressure.TGA was performed with a Netzsch TG 209 instrument. The nanoparticles (ca. 10 mg) were heated under a nitrogen flow at a rate of 10 C/min to 600 C.TEM was performed using a Philips EM 301 electron microscope. A drop of the toluene reaction mixture containing the colloidal metal particles was placed onto a carbon film supported by a copper grid. After evaporation of the solvent, the samples were inserted into the electron microscope. The undrawn and drawn polyethylene/silver nanocomposites (containing 4 wt.-% nanoparticles) were cut at low (±80 C) temperatures using a diamond knife; the thin slices were collected on a copper grid and used for TEM.Polarized UV-vis spectra were recorded with a Perkin-Elmer Lambda 900 instrument, fitted with motor-driven Glan±Thomson polarizers. The nanocomposites were sandwiched between two quartz glass slides and coated with a small amount of silicone oil in order to minimize light scattering at the film surfaces [22].

Self‐assembled Electroactive Phosphonic Acids on ITO: Maximizing Hole‐Injection in Polymer Light‐Emitting Diodes

“…A. Bardecker et al / Self-assembled Electroactive Phosphonic Acids on ITO dipole of self-assembled molecules has been shown to affect the work function of ITO in extensive previous studies. [3,4,14,20,21] All of the triarylamine-based molecules have dipole moments pointing away from the ITO surface, as shown in Figure 7. A dipole moment (m) oriented away from the surface would effectively shift the vacuum level outside of the anode down, decreasing the work function (F) of the ITO anode and increasing the barrier to hole-injection.…”

“…In addition, molecular self-assembly can occur spontaneously from a solution or gas [1] enabling the formation of highly ordered functional monolayers with few processing steps. Selfassembled monolayers (SAMs) have already shown their ability to promote crystallization of small molecules, [2] form cohesive dielectric films by layer-by-layer self-assembly, [3] change the work function of inorganic conductors, [4,5] change the morphology and orientation of organic layers, [6,7] and passivate electronic traps. [8] The need for precise control of the electrode/organic semiconductor interface is particularly apparent in the field of polymer light-emitting diodes (PLEDs), where large differences between the work function of the commonly used indium tin oxide (ITO) anode and the highest occupied molecular orbital (HOMO) level of the organic semiconductor limit hole-injection and can lead to high turn-on voltage, low brightness, and low efficiency.…”

Modification of ITO surface using aromatic small molecules with carboxylic acid groups for OLED applications

“…This concept has been extensively studied and reported in the previous publications [36][37][38][39][40]. Charge transfer from the COOH head group of MPPBA molecule to ITO surface leads to the formation of C-O bond due to electrostatic interaction between positively charged ITO surface and the delocalized electrons in the oxygen atom of carboxylate group in MPPBA molecule [41].…”

Interfacial Microstructure Function in Organic Light-Emitting Diodes: Assembled Tetraaryldiamine and Copper Phthalocyanine Interlayers

“…These approaches involve deposition onto ITO of nanoscale layers of various organic acids, [6±8] copper phthalocyanine, [9] dielectrics, [10] or thicker (30±100 nm) layers of polyaniline [11±13] or a polythiophene (PE-DOT), [14] all resulting in enhanced luminous performance. Explanations for these phenomena are diverse and include altering interfacial electric fields, [1,3,6±8,15,16] balancing electron/hole injection fluence, [10,17] confining electrons in the emissive layer, [18] reducing injected charge back-scattering, [19] and moderating anode Fermi level±HTL HOMO energetic discontinuities. [14,20] This diversity of proposed mechanisms accurately reflects the complexity of interactions at OLED interfaces and, in many cases, the lack of necessary microstructural information.…”